The pervasive decline in ankle power generation, even in otherwise healthy older adults, plays a central role in the deterioration of walking abilit with age. However, muscle strengthening alone generally fails to significantly improve the walking ability of older adults, suggesting that factors other than sarcopenia contribute to the decline in plantarflexor power with age. I propose to investigate the relevance of age-related changes in Achilles tendon mechanics and their implications for walking ability in older adults. The premise of this study is that the aging Achilles tendon exhibits altered in vivo behavior that greatly diminishes the ability to optimally coordinate the ankle plantarflexor muscles during the push-off phase of gait. Preliminary data suggest that the superficial and deep Achilles tendon fascicles exhibit distinctly different displacements, likely due to inter- fascicle sliding, which ay be critical for optimally coordinating the forces from the individual plantarflexor muscles. Moreover, these data show evidence that advancing age brings much more uniform tendon tissue motion, which may arise from inter-fascicle adhesions within the Achilles tendon. The overall hypothesis of this research is that greater fascicle adhesions within the Achilles tendon of older adults bring more uniform tendon deformations that reduce the capacity for optimally coordinating the individual plantarflexor muscles, thereby compromising plantarflexor power and thus gait performance. The approach utilizes novel advances in imaging biomechanics and computational modeling to investigate age-related changes in Achilles tendon mechanics and function, and to predict the ramifications for walking ability in older adults. Following are the specific aims. Aim 1 will use an advanced ultrasound elastography approach to determine if advancing age is accompanied by greater Achilles tendon strain uniformity, a potentially important factor to consider as a mechanism for age- related walking ability limitations. Aim 2 will use an innovative combination of ultrasound elastography and electrical muscle stimulation to reveal if greater fascicle adhesions within the Achilles tendon of older adults bring more uniform deformations between the deep and superficial portions of the tendon. Finally, Aim 3 will incorporate the findings from Aims 1 and 2 into muscle-actuated forward dynamic simulations to predict changes in plantarflexor function during walking in older adults. This research will investigate for the first time the age-related changes in localized Achilles tendon strain distributions (Aim 1), their underlying mechanisms (Aim 2), and the ramifications for walking ability in older adults (Aim 3). Successful completion of these aims will contribute to an enhanced scientific understanding of age-related declines in walking ability and point to specific opportunities where more appropriate therapies could prolong independence and thus enhance quality of life in older adults.